[zooom]

is there a light bulb in the storage space?....if so then get one of those adapters from home depot that has a plug and allows the bulb to screw in and then plug a 6 foot extension cord to a power strip and then plug in the necessary amount of Battery Tender's......if the owner of the storage space says anything...then remove it all....but if I were you....that'd be my reccomendation....also consider the types of batteries there are out there instead of the liquid lead cell...there are Gel Cell and AGM batteries.....this company though seels a great charger with their batteries as an option and I got my most recent battery from them and a charger and I have used it to charge a couple of batteries so far and it goes to auto trickle!
http://www.westcobattery.com/index.html

 

[zooom]

well....I would certainly hate to have to drag a battery along with all the other gear to the bike everytime I'd want to ride...and what about taking the right battery for the right bike?...what if you change your mind as to which bike you want to ride before you get there?...maybe look into getting a big fat deep cycle marine battery and put it in a small lockbox to leave there with the bikes with some baby jumper cables and jump a bike there when you need to off the marine battery and then take that home periodically to charge it!

 

[zooom]

Optimate, fit a jack plug to your bike and leave it connected whenever you're not riding and forget about Battery issues.

http://www.accumate.co.uk/it010003.html

that's exactly what I got when I got my new AGM battery from these folks...

http://www.westcobattery.com/index.html